Semiconductor device and a method for fabricating the same

ABSTRACT

In a plastic molded semiconductor device in which inner leads overlap a semiconductor chip in a molded plastic body, the width of the chip may be close to the width of the plastic body without a decrease in the high resistance of the inner leads to the pull out thereof from the plastic body, and the layout of the inner leads may be unrestricted since the inner leads may occupy the region above the chip.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and morespecifically to a plastic molded semiconductor device and a method forfabricating the same.

2. Description of the Prior Art

A plastic molded semiconductor device, for example, a plastic moldedintegrated circuit, is known and is fabricated by assembling andinterconnecting a semiconductor chip on a substrate or lead frame andmolding the entire structure in plastic, with the exception of theleads, to form the body of the component. In this type of fabrication, asingle lead frame made from a metal ribbon and comprising a stage andleads is used, with the result that the leads do not overlap the stageon which the semiconductor chip is mounted.

Although the size of a semiconductor chip tends to be increased, thesize of a semiconductor device should be a predetermined size, a rowspace size, or, preferably, as small as possible so that it can bepacked more densely. In a semiconductor device, however, leads must beembedded in plastic in more than a fixed length in order to obtain arequired pull out resistance, which limits the maximum size of a stageon which a semiconductor chip is mounted and, therefore, limits themaximum size of a semiconductor chip mounted on the stage if the size ofa semiconductor device is predetermined.

Further, recent large-scale integrated circuits tend to have electrodesarranged only on two longitudinally opposed edges of a semiconductorchip. Therefore, it is difficult to form a layout of leads adoptedsuitable for this arrangement of electrodes, and it is particularlydifficult if the width from the outer edge of a plastic body to bemolded to near the stage on which a semiconductor chip is mounted isnarrow.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a plastic moldedsemiconductor device in which a larger semiconductor chip can be mountedand to form a layout of leads which is less restricted than that of theprior art case.

Another object of the present invention is to provide a method forfabricating such a semiconductor device.

These and other objects, features, and advantages of the presentinvention are accomplished by providing a semiconductor device having amolded plastic body, the semiconductor device comprising: the moldedplastic body; a stage in the molded plastic body; a semiconductor chiphaving electrodes and being mounted on the stage in the molded plasticbody; and a plurality of leads each consisting of an inner lead portionembedded in the molded plastic body and an outer lead portion extendingoutside of the molded plastic body, at least one of the leadsoverlapping the stage, and the leads and the electrodes of thesemiconductor chip being interconnected.

The leads preferably overlap not only the stage but also thesemiconductor chip so as to create a larger area for the inner leads. Inthis case, an insulating layer, preferably a polyimide or silicone film,may be inserted between the leads and the semiconductor chip except forthe electrodes of the semiconductor chip. The insertion of an insulatinglayer is preferable not only for electrically insulating thesemiconductor chip from the wires or leads but also for preventingdamages resulted from the direct contact of the inner leads to thesurface of the semiconductor chip during the wire bonding process.

The present invention also provides a method for fabricating asemiconductor device, including the steps of: mounting a semiconductorchip having electrodes onto a stage; fixing the stage to a lead framehaving a plurality of leads each consisting of an inner lead portion andan outer lead portion in such a manner that at least one of the leadsoverlaps the stage; interconnecting the electrodes of the semiconductorchip and the inner lead portions of the leads; molding the structure ofthe stage, the semiconductor chip, and the inner lead portions of theleads in plastic; and separating the thus-molded structure from the restof the lead frame.

The stage may be fixed to the lead frame by welding, such as spotwelding or heat pressure welding, or bonding, e.g., with an adhesive,caulking, or the like.

For fixing the stage to the lead frame, a stage alone, a tab comprisinga stage and arms, or another lead frame comprising a stage may be used.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features, and advantages of the present invention aredescribed with reference to the accompanying drawings.

FIG. 1 is a sectional view of a prior art semiconductor device.

FIG. 2 is a plan view of a prior art assembly of a semiconductor devicejust before molding is effected.

FIG. 3 is a sectional view of a semiconductor device according to thepresent invention.

FIG. 4a is a plan view of a lead frame used in combination with a tab,and FIGS. 4b and 4c are a plan view and a sectional view, respectively,of a tab.

FIG. 5 is a plan view of an assembly before molding.

FIG. 6a is a plan view of a capacitor stage, FIG. 6b is a plan view of alead frame used in combination with the capacitor stage of FIG. 6a, andFIG. 6c is a partial sectional view of the assembly of the capacitorstage, a chip, and the lead frame.

FIG. 7a and FIG. 7b are plan view and sectional view taken along theline VIIA--VIIA in FIG. 7a of a lead frame, and FIG. 7c is also a planview of a lead frame, the lead frame of FIG. 7a and the lead frame ofFIG. 7c being used in combination.

FIGS. 8a and 8b are cross-sectional view of and a longitudinal sectionalview of a mold including the assembly of two lead frames and a chip.

FIGS. 9 and 10 are sectional views of further semiconductor devicesaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing the present invention, the prior art is described withreference to FIGS. 1 and 2. In FIG. 1, a semiconductor device 1comprises a plastic body 2, a stage 3, a semiconductor chip 4, aplurality of leads 5, and wires 6. The stage 3 and the leads 5 consistof a single lead frame made from a metal ribbon by stamping or etchingit and, therefore, the leads 5 do not overlap the stage 3. FIG. 2illustrates an example of patterns of an assembly of a prior art leadframe and a semiconductor chip. In FIG. 2, the same parts as in FIG. 1are denoted by the same reference numerals. Reference numeral 7 denotescradles, 8 dam bars, and 9 pinch bars, the cradles 7 and a part of pinchbars 9 near the cradle 7 and the dam bars 8 except for the leads 5 beingremoved from a finished semiconductor device.

FIG. 3 illustrates a semiconductor device 11 according to the presentinvention. In the figure, reference numeral 12 denotes a plastic body,13 a stage, 14 a semiconductor chip, 15 a plurality of leads, 16 wires,and 20 electrodes of the semiconductor chip 14. In this device, theleads 15 overlap the stage 13, as well as the semiconductor chip 14, sothat the leads 15 have enough depth in the plastic body 12 to give thema high pull out resistance and so that a wide region above thesemiconductor chip 14 can be used for extending each of the leads 15 tonear a desired electrode 20 of the semiconductor chip 14 irrespective ofthe size of the semiconductor chip 14, i.e., even if it is a size closeto the width of the plastic body 12. Further, in this construction of asemiconductor device, the distance between the leads 15 and theelectrodes 20 of the semiconductor chip 14 can be decreased to a minimumso that the expensive wires 16 of, e.g., gold can be shortened and thetime period for wire bonding can be greatly shortened, resulting in areduction of production costs.

FIG. 4a illustrates a lead frame according to the present invention. Alead frame 21 is made from a metal ribbon of, e.g., 42-alloy bystamping, etching, etc. and contains several parts, each of which isused for fabricating one semiconductor device. Below, a part of a leadframe from which one semiconductor device is fabricated is described.The lead frame 21 comprises cradles 17 running along the twolongitudinal sides of the lead frame itself and provided with alignmentholes, a plurality of leads 15 consisting of an inner lead portion 15aand an outer lead portion 15b, dam bars 18 tying the leads 15 to eachother and to the cradles 17, and a receiving portion 22 near the cradles17 for fixing a tab. The lead frame 21 does not include a stage on whichthe semiconductor chip 14 is mounted since both the leads 15 and a stagecannot be formed from a single metal ribbon due to the overlappingthereof in a semiconductor device according to the present invention.

FIGS. 4b and 4c illustrate a tab 23 comprising a stage 13 and two arms24. The arms 24 are bent to differentiate the levels of the stage 13 andthe ends of the arms 24.

In the fabrication of a semiconductor device, referring to FIGS. 3 to 5,a semiconductor chip 14 having electrodes 20 is bonded onto a stage 13of a tab 23 with, e.g., silver paste. An insulating film 25, e.g., apolyimide or silicone tape, 50 λm to 200 λm thick is bonded onto all ofthe top surface of the semiconductor chip 14 except for the electrodes20. The insulating film 25 is preferably heat resistant since it will besubjected to heat in later processes. This insulating film 25 may servenot only as an electrical insulating layer but also as a protectivelayer for a semiconductor chip against damages resulted from directcontacts of leads during wire bonding process. Next, the tab 23 on whichthe chip 14 is mounted is fixed to a lead frame 21 by welding or bondingthe arms 24 of the tab 23 and the tab-receiving portions 22 of the leadframe 21. The type of welding is preferably spot welding or heatpressure welding. An adhesive such as polyimide may be used as a bond.The electrodes 20 of the semiconductor chip 14 and the inner leadportions 15a of the leads 15 are then interconnected by means of, e.g.,gold wire bonding.

The thus-assembled and interconnected structure is illustrated in FIG.5. As can be seen from FIG. 5, the ends of some inner lead portions 15aoverlap the stage 13 and the semiconductor chip 14. In FIG. 5, thoughthe semiconductor chip 14 has electrodes 20 arranged along two sidesthereof and making a row perpendicular to the row of the leads 15, allof the ends of the inner lead portions 15a extend to near the electrodes20 of the chip 14 since the leads 15 can occupy the region above thechip 14. This possibility of an unrestricted layout of the inner leadportions is a great advantage of the present invention.

The thus-assembled and interconnected structure comprising the leadframe 21, the tab 23, the chip 14, and the wires 16 is set in antransfer mold comprising on upper half and a lower half. The mold has amold space having peripheries along the dam bars 18 and near the cradles17 so that a structure consisting of the stage 13, the chip 14, theinner lead portions 15a of the leads 15, and the wires 16 can be moldedin plastic, e.g., epoxy resin. After the molding is carried out, thecradles 17 and the dam bars 18, but not the portions of the leads 15,are removed with a press so as to separate the molded plastic body andthe outer lead portions 15b, corresponding to an individual plasticmolded semiconductor device. Conventional processes such as metalplating, bending the outer lead, etc. may then be carried out.

In accordance with the present invention, a semiconductor device havinga molded plastic body 6.6 mm wide can contain a semiconductor chip 5.5mm wide and can have a high resistance to withdrawal of the leads. Incomparison, a prior art semiconductor device having a molded plasticbody 6.6 mm wide can contain only a semiconductor chip 4 mm wide at themaximum since the depth of the inner leads and the space between theinner leads and the stage must be at least 1 mm and 0.3 mm,respectively.

FIGS. 6a, 6b, and 6c illustrate another embodiment of the presentinvention in which a stage on which a semiconductor chip is mounted is acapacitor. For example, a capacitor 40 is made of a ceramic plate 41 of,e.g., alumina provided with electrodes 42. The capacitor 40 is fixed toa lead frame 43 at the point indicated by the broken lines in FIG. 6bafter a semiconductor chip 14 is mounted onto the capacitor 40. To fixthe capacitor 40 to the lead frame 43, the lead frame 43 may havereceiving portions 44 which do not overlap the inner lead portions ofthe leads. Thus, the capacitor 40 may be directly fixed to the leadframe 43. The capacitor itself is proposed as a stage for improving thecharacteristics of a semiconductor chip mounted on the stage.

An armless metal stage may be used in combination with a lead framesimilar to that of FIG. 6b.

FIGS. 7a and 7b illustrate a further embodiment of the present inventionin which two lead frames are used for fabricating a semiconductordevice. In this embodiment, a stage 13 is formed as a part of a firstlead frame 50 and is tied to cradles 17 with pinch bars 19. A secondlead frame 51 is similar to the lead frame 21 in FIG. 4a except that thetab-receiving portions are removed.

The assembling and interconnecting processes are similar to those in theembodiments described before except that the stage 13 is fixed to thesecond lead frame 51 by fixing the two lead frames 50 and 51 to eachother by, preferably, welding some portions of the cradles 17. In themolding process, a particular mold adopted for receiving two lead framesis necessary. FIGS. 8a and 8b illustrate a cross-sectional and alongitudinal sectional view, respectively, of such a particular moldincluding an assembly of two lead frames and a semiconductor chip. Ascan be seen from FIGS. 8a and 8b, a mold 52 comprising an upper half 52aand an lower half 52b holds a single lead frame of the second lead frame51 at the dam bars 18 thereof and has apertures for receiving thelaminations of two cradles. A mold space 53 contains the assembly of astage, a chip, and inner lead portions of leads and is connected by agate 54 to a runner 55 which is connected to a cull (not shown), fromwhere transfer plastic material is fed into the mold space 53.

FIG. 9 illustrates still another embodiment of the present invention inwhich an insulating layer is inserted between the semiconductor chip anda part of the leads, but not on or above the entire surface of thesemiconductor chip except for the electrodes.

In a semiconductor device 61 in FIG. 9, insulating films 62 are insertedbetween the semiconductor chip 14 and the end portions of the lead 15(15a). This insertion of the insulating film may be effected by adheringthe insulating films 62 to the top surface of the semiconductor chip 14or to the end portions of the leads 15 (15a). FIG. 9 illustrates theformer case. This insulating film can also protect a semiconductor chip14 from damages resulted from direct contacts of the end portions ofleads 15 (15a) when wire bonding is effected.

FIG. 10 illustrates still a further embodiment of the present inventionin which inner leads 15a overlap a stage 13 but not a semiconductor chip14. In this semiconductor device 63, an insulating layer 64 is insertedbetween the inner leads 15a and the stage 13.

We claim:
 1. A semiconductor device having a molded plastic body, saidsemiconductor device comprising:a semiconductor chip having a majorsurface and having electrodes formed on said major surface, saidsemiconductor chip being encapsulated in the molded plastic body; athin, flat, self-supportive metal stage onto which said semiconductorchip is bonded so that said major surface is opposite to and faces saidmetal stage, said stage being encapsulated in said molded plastic body;a plurality of leads, each comprising an inner lead end portion embeddedin the molded plastic body and an outer lead end portion extendingoutside of the molded plastic body, said inner lead end portions of theleads being spaced apart from said semiconductor chips, said inner leadend portion of at least one of the leads overlapping said stage andextending over said major surface of said semiconductor chip; andinterconnection wires interconnecting each of said inner lead endportions and each of said electrodes of said semiconductor chip.
 2. Asemiconductor device having a molded plastic body, said semiconductordevice comprising:a semiconductor chip having electrodes and beingencapsulated in the molded plastic body; a thin, flat, self-supportivemetal stage onto which said semiconductor chip is bonded, said stagebeing encapsulated in said molded plastic body; a plurality of leads,each comprising an inner lead end portion embedded in the molded plasticbody and an outer lead end portion extending outside of the moldedplastic body, said inner lead end portions of the leads being spacedapart from said semiconductor chips, said inner lead end portion of atleast one of the leads overlapping said stage and said semiconductorchip; and interconnection wires interconnecting each of said inner leadend portions and each of said electrodes of said semiconductor chip,wherein said electrodes are arranged along at least one side of saidsemiconductor chip and said inner lead end portion of at least one ofsaid leads extends over said semiconductor chip to a position adjacentto inner sides of said electrodes relative to said semiconductor chip.3. A semiconductor device according to claim 2, wherein an insulatingfilm covers an entire top surface of the semiconductor chip except forthe electrodes thereof.
 4. A semiconductor device according to claim 2,wherein an insulating film exists in part of the space between thesemiconductor chip and only the inner lead end portions of the leads. 5.A semiconductor device according to claim 4, wherein the insulating filmbetween the semiconductor chip and the inner lead end portions of theleads is adhered to the end portion of the leads.
 6. A semiconductordevice according to claim 4, wherein the insulating film between thesemiconductor chip and the inner lead end portions of the leads isadhered to the semiconductor chip.
 7. A semiconductor device accordingto claim 3 or 4, wherein the insulating film is of polyimide.
 8. Asemiconductor device according to claim 3 or 4, wherein the insulatingfilm is silicone.
 9. A semiconductor device according to claim 2,wherein the inner lead end portions of the leads are arranged in atleast one row, said electrodes of the semiconductor chip being arrangedalong two sides of the semiconductor chip forming rows of electrodeswhich run perpendicular to at least one row of leads.
 10. Asemiconductor device having a molded plastic body, said semiconductordevice comprising:a semiconductor chip having electrodes and beingencapsulated in the molded plastic body; a thin, flat, self-supportivemetal stage onto which said semiconductor chip is bonded, said stagebeing encapsulated in said molded plastic body; a plurality of leads,each comprising an inner lead end portion embedded in the molded plasticbody and an outer lead end portion extending outside of the moldedplastic body, said inner lead end portions of the leads being spacedapart from said semiconductor chips, said inner lead end portion of atleast one of the leads overlapping said stage and said semiconductorchip; and interconnection wires interconnecting each of said inner leadend portions and each of said electrodes of said semiconductor chip,wherein the stage is a capacitor made of a ceramic plate having twoelectrodes, each electrode being interconnected to one of the leads.